S. T. Pratt

Electric field effects in the near‐threshold photoionization spectrum of nitric oxide

Zero‐kinetic‐energy–photoelectron spectroscopy (ZEKE‐PES) and time‐of‐flight mass spectrometry are used to investigate the near‐threshold, two‐color photoionization of NO via the A 2Σ+, v=0 level. Pulsed‐field ionization of Rydberg states within 20 cm−1 of the ionization threshold is shown to proceed via a diabatic mechanism. Particular emphasis is given to the effect of predissociation on the signal produced by delayed field ionization. A small (∼3 V/cm) dc electric field is shown to produce a dramatic decrease in the field ionization signal in both the NO+‐ion and ZEKE‐electron channels. This decrease is thought to be due to an increase in the predissociation rate caused by the dc electric field. The implications of these experiments for mass‐analyzed threshold ionization are discussed.

Photoelectron studies of resonant multiphoton ionization of CO via the A 1Π state

We report measurements of multiphoton ionization of CO involving a three photon resonance to the v=1–3 levels of the CO A 1Π state followed by the absorption of either two (v=3) or three (v=1,2) additional photons to reach the ionization continua. In one measurement, the CO+ ion intensity is measured as a function of wavelength, yielding the rotational structure of the intermediate resonant state. In a second measurement, the laser frequency is set at the v=1–3 bandheads and the kinetic energy spectra of the ejected electrons are measured. The observed vibrational branching ratios in these photoelectron spectra do not follow the pattern predicted by the Franck–Condon overlap between the intermediate A 1Π state and the ionization continua. Several possible causes for this (not unexpected) deviation from Franck–Condon behavior are discussed.

Photoelectron studies of resonant multiphoton ionization of molecular nitrogen

Photoelectron studies of three photon resonant, four photon ionization of N2 via the b 1Πu, v′=0–5, c 1Πu, v′=0, 1, and c′ 1Σ+u, v′=0, 1 levels are presented. A qualitative analysis of the observed photoelectron spectra in terms of homogeneous perturbations among the b 1Πu, c 1Πu, o 1Πu states and among the b′ 1Σ+u, c′ 1Σ+u, and e′ 1Σ+u states is partially successful. However, several of the photoelectron spectra exhibit marked deviations from the expected behavior. These deviations are discussed in terms of more complex interactions among the intermediate levels, and in terms of the dynamics of excited state photoionization. In addition, the N2 a 1Πg, v′=0–5 levels have been observed in two photon resonant, four photon ionization, and the results are compared with the results of earlier two‐photon laser induced fluorescence studies.

Photoionization dynamics of excited molecular states. Photoelectron angular distributions and rotational and vibrational branching ratios for H2 C 1Πu, v=0–4

Photoelectron angular distributions following three photon resonant, four photon (3+1) ionization of H2 via the C 1Πu, v′=0–4 ← X 1Σ+g, v″=0 Q(1) transitions are reported. The observed angular distributions are generally more isotropic for v+≠v′ than for v+=v′. Photoelectron spectra obtained along the polarization axis of the laser following (3+1) ionization via the C 1Πu, v′=4 ← X 1Σ+g, v″=0 R(0) and R(1) transitions are also reported. These spectra are rotationally resolved and exhibit strongly v+‐dependent rotational branching ratios. The comparison of the angular distribution data with available theoretical calculations indicates good agreement for some transitions and poor agreement for others, suggesting the need for substantial progress in understanding the photoionization dynamics of even the simplest excited molecular states.

Velocity map imaging of the photodissociation of CF3I: Vibrational energy dependence of the recoil anisotropy

The photodissociation of jet-cooled CF3I into CF3+I(2P3/2) and CF3+I*(2P1/2) has been investigated between 304 and 277 nm by using velocity map ion imaging. The two-dimensional images provide detailed information on the partition of available energy into kinetic and internal energy of the photofragments. Vibrational structure with spacing of 695±100 cm−1 is resolved in both I and I* images, indicating excitation of the umbrella mode ν2 of the CF3 photofragment. The fragment recoil anisotropies β(I) and β(I*) are determined as a function of the excitation wavelength and their variations are interpreted in terms of the crossing between the 3Q0 and 1Q1 dissociative electronic states. The high-resolution images allow the determination of the variation of the anisotropy parameter β as a function of the vibrational level of CF3 fragment, and provide a complementary method for the determination of the C–I bond energy. The vibrational dependence of the anisotropy values is discussed in terms of final-state intera...

X-ray imaging and microspectroscopy of plants and fungi.

X-ray fluorescence microscopy and microspectroscopy with micrometre spatial resolution and unprecedented capabilities for the study of biological and environmental samples are reported. These new capabilities are a result of both the combination of high-brilliance synchrotron radiation and high-performance X-ray microfocusing optics and the intrinsic advantages of X-rays for elemental mapping and chemical-state imaging. In this paper, these capabilities are illustrated by experimental results on hard X-ray phase-contrast imaging, X-ray fluorescence (XRF) imaging and microspectroscopy of mycorrhizal plant roots and fungi in their natural hydrated state. The XRF microprobe is demonstrated by the simultaneous mapping of the elemental distributions of P, S, K, Ca, Mn, Fe, Ni, Cu and Zn with a spatial resolution of approximately 1 x 3 micron and with an elemental sensitivity of approximately 500 p.p.b. Microspectroscopy with the same spatial resolution is demonstrated by recording near-edge X-ray absorption (XANES) spectra of Mn at a concentration of approximately 3 p.p.m.

Two photon resonant, four photon ionization of CO via the A 1Π state with photoelectron energy analysis

We report measurements of photoelectron spectra from resonantly enhanced four photon ionization of CO via a two photon transition to the R‐branch heads of the A 1Π, v=1–3 ← X 1Σ+, v=0 bands. In addition, we present preliminary data on the rotational state dependence of the photoelectron spectra. The results are compared with our earlier measurements of the photoelectron spectra obtained in the six photon ionization of CO via a three photon resonant transition to the same intermediate levels. The observed vibrational branching ratios in the photoelectron spectra differ both from those observed in the earlier study and from the pattern predicted by the Franck–Condon overlap between the intermediate A 1Π state and the ionization continua. It is shown that accidental resonances with the J 1Σ+ state at the three photon energy account for the observed photoelectron spectra obtained at the wavelengths of the A 1Π, v=2,3 R‐branch bandheads. Finally, we present conclusive evidence for the production of electronica...

Photoionization of vibrationally hot CH3 and CF3

Vibrationally hot CH(3) and CF(3) were produced by the 266 nm photodissociation of CH(3)I and CF(3)I, respectively, and probed by single-photon ionization at 118 nm. Comparison of the ion images of the CH(3) and CF(3) fragments with those of the complementary I atoms, and with previous measurements of the product branching fractions, allowed the determination of the relative photoion yields as a function of the vibrational energy of the molecular radical. Some general ideas about the internal-energy dependence of photoionization cross sections are also discussed.

Vibrational autoionization in polyatomic molecules

The vibrationally autoionizing Rydberg states of small polyatomic molecules provide a fascinating laboratory in which to study fundamental nonadiabatic processes. In this review, recent results on the vibrational mode dependence of vibrational autoionization are discussed. In general, autoionization rates depend strongly on the character of the normal mode driving the process and on the electronic character of the Rydberg electron. Although quantitative calculations based on multichannel quantum defect theory are available for some polyatomic molecules, including H3, only qualitative information exists for most molecules. This review shows how qualitative information, such as Walsh diagrams along different normal coordinates of the molecule, can provide insight into the vibrational autoionization rates.

Photoionization of hot radicals: C2H5,n-C3H7, and i-C3H7

The combination of ion-imaging and vacuum-ultraviolet (vuv) single-photon ionization is used to study the internal energy dependence of the relative photoionization yields of the C(2)H(5),n-C(3)H(7), and i-C(3)H(7) radicals following the 266 nm photodissociation of the corresponding alkyl iodides. The comparison of the ion images obtained by vuv photoionization of the radical with those obtained by two-photon-resonant, three-photon ionization of the complementary I (2)P(32) and I*(2)P(12) atoms allows the extraction of the internal energy dependence of the cross sections. Factors influencing the appearance of the ion images in the different detection channels are discussed, including the secondary fragmentation of the neutral radicals, Franck-Condon factors for the photoionization process, and the unimolecular fragmentation of the parent photoions.